In a device relating to heat, it is often required to minimize power inputted thereinto and at the same time to raise temperature as much and rapidly as possible. To achieve this object, it is necessary to make as small as possible a heat conductance as well as a heat capacity of a section where temperature rises. The best way to achieve the object is to realize a thin film construction in which the section where temperature rises is floating, namely so-called the floating thin film construction. The present applicant proposed a micro-heater based on the floating thin film construction in the patent application for an "electric heater" (Japanese Patent No. 1398241), and since then micro-heaters based on this floating thin film construction or various types of heat device using the floating thin film construction have been developed and put into practical use.
Conventionally such a material as amorphous insulating thin film or metallic thin film including oxidized silicon film and silicon nitride film has been used for the floating thin film construction, but as it is easier to directly form a device in or on the single crystal thin film, development of a floating single crystal thin film construction has been aspired. There has been a method of preparation of a floating single crystal thin film made of heavily boron doped Si making use of the fact that, when boron is doped to Si, the resultant thin film is little affected by such an anisotropic etchant such as a KOH solution or a hydrazine solution, but as the heavily boron deposed Si has an extremely low resistance, it is difficult to directly form a semiconductor device in or on the heavily doped thin film, so that it has been necessary to deposit an epitaxial growth thin film with lightly doped impurities and form a semiconductor device in or on the thin film.
Also as a way of preparation of a floating single crystal thin film made of lightly doped impurity, there has been a method of depositing an epitaxial growth thin film with lightly doped impurities in a Si substrate having a different conduction type from that of the epitaxial growth thin film (for instance, the substrate must be of p-type, if the epitaxial growth thin film is of n-type) and then carrying out etching while applying a voltage to the anisotropic etchant so that a p-n junction formed there is biased in the reverse direction. In this method, however, it is necessary to use an expensive epitaxial growth substrate, and if a partial floating single crystal thin film should be doped, it is necessary to establish an electrical connection to the section, which is disadvantageous.